Maximizing Slowly Digested Starch in an Expanded Sorghum-maize Extruded Food Using Response Surface Methodology

Abstract

Sorghum is a grain with potential for developing foods with slowly digested starch, of benefit to healthy glucose metabolism. Response surface methodology (RSM) was used to determine the effects of total moisture in barrel (22–25%), final barrel zone temperature (115–140°C), and sorghum flour in dry mix (30–80%) during extrusion of sorghum-maize flour, on extrudate slowly digested starch (SDS) and expansion ratio. SDS level had a negative linear association (p = 0.007) with final barrel zone temperature and a positive linear association (p < 0.001) with sorghum level. Expansion ratio had a quadratic association with final barrel zone temperature (p = 0.002) and moisture in barrel (p < 0.001). A linear model described the combined effects of extrusion parameters on SDS levels (R2 = 68.23%), while a quadratic model described their effects on expansion ratio (R2 = 75.24%). SDS level was positively associated with both polyphenolic level (r = 0.622, p = 0.001) and antioxidant capacity (r = 0.668, p = 0.001). The validated RSM model indicated that 22% total moisture in barrel, 115 °C final barrel temperature zone, and 74.67% sorghum in dry mix were optimum settings to deliver maximum levels of SDS with adequate expansion ratio. This is the first report of the optimisation of SDS level in a sorghum based extrudate. These findings demonstrate the potential of sorghum for the development of extruded snack foods with elevated levels of both SDS and antioxidant capacity.